One of laser applications is cutting. The following is typical cutting by laser. For example, a part to cut in an object to be processed such as a semiconductor wafer or glass substrate is irradiated with laser light having a wavelength absorbed by the object, and melting by heating is advanced by the laser light absorption from the front face to rear face of the object in the part to cut, so as to cut the object. However, this method also melts the surroundings of a region which becomes a part to cut in the front face of the object. As a consequence, in the case where the object to be processed is a semiconductor wafer, there is a fear of semiconductor devices positioned near the above-mentioned region among those formed on the front face of the semiconductor wafer melting.
Examples of methods of preventing the front face of such an object to be processed from melting include laser cutting methods disclosed in Japanese Patent Application Laid-Open Nos. 2000-219528 and 2000-15467. The cutting methods of these publications cause laser light to heat a part to cut in the object to be processed, and then cool the object, so as to generate a thermal shock at the part to cut in the object, thereby cutting the object.